I’m a bit stumped. Can you figure this out?

Hello everybody. My name is John and I’m a contractor who is stumped with an issue. I recently installed a reliance controls 10 circuit manual transfer switch (model 310crk pro/tran 2). The switch seems to be working fine when in line mode. All circuits to the house are fine and running ok. All 10 circuits are also single pole so no 240 needed. Client has a firman 4000W inverter generator (model.WH03242) with a standard three prong RV plug. Looking in the manual this model has a floating neutral so should be compatible with the switch. Client has a bridged adapter that converts to a four prong twist lock that feeds into the inlet box.

We fire up the generator, leave all in the line position plug in the electrical wire that goes to the transfer switch inlet into the generator and it goes into overload mode and cuts off all outlets to the generator. Have tried the same with the transfer switch in the off position and again cuts power. This generator is not gfci protected and all the circuits wired to the transfer switch are regular single breakers (no afci or gfci).

some of the testing I did was to disconnect one breaker f the hot legs that goes into the transfer switch. So I disconnected the red hot coming from the inlet to the switch from the red of transfer switch and it trips. Did the same with the black and it trips. Disconnect app wires and am able to read 120V across the red and neutral and black and neutral so I know power is coming coming as it should.

Does anybody know what can be causing this?